In a medium bowl, whisk together flours, poppy seeds, baking powder and salt.

In a large mixing bowl, add zest and 2/3 cup sugar - rub the two together with your fingers until the lemon zest is evenly distributed and the sugar takes on a light yellow hue. Add butter and beat until light and fluffy. Add eggs, one at a time, beating until well combined after each. Mix in vanilla. Add half of the dry ingredients - mix just until combined, then mix in milk. Add remaining dry ingredients and stir just until combined.

Scoop the mixture into a 9" x 5" loaf pan coated with nonstick spray. Bake until a toothpick placed in the center comes out mostly clean with a few moist crumbs attached, about 45 to 55 minutes.

Meanwhile, in a small saucepan, stir together remaining 1/3 cup sugar and lemon juice. Bring to a boil, stirring, and let cook for 30 seconds. Remove from the heat. Measure out 2 tablespoons of the syrup and place in a small bowl. Set both aside.

As soon as the cake come out from the oven, use a toothpick to poke wholes all over the top. Pour the syrup that is still in the saucepan all over the cake. Place the pan on a rack and let cool completely. Turn the cake out of the pan and brush all over with the reserved 2 tablespoons syrup.